The present invention relates to optical amplifier fiber modules and optical amplifier fibers.
Optical fibers have attracted a great deal of attention in the telecommunication industry because of their ability to carry larger quantities of information with longer distances between signal boosters than is possible using conventional metal wires. One reason for this is that in conventional wires, such as copper wire and coaxial cable, attenuation increases exponentially with signal frequency, making high-speed transmission over long distances impractical. While light signals in optical fibers also suffer from attenuation as a light signal travels along the fiber, it is significantly less than the loss found in conventional wires. In addition, attenuation in SiO2 fibers is independent of signal frequency. Both of these advantages help the optical fiber handle more information over larger ranges than is possible in conventional wires.
In long fiber spans, attenuation may weaken the signal to the point where it cannot excite a photodetector in the receiver. The result is a failed transmission. To cope with this problem, the light signal can be amplified along the length of the fiber span. In optical communication networks, research has focused on two approaches to amplify light signals: repeaters and optical amplifiers.
A repeater is a device that receives an optical signal, converts the optical signal to an electrical signal, amplifies the electrical signal, and converts the amplified electrical signal back to an optical signal. As will be appreciated, repeaters, are time consuming and lossy. An optical amplifier, on the other hand, amplifies the optical signal without the need for conversion to an electrical signal. Other benefits of optical amplifiers include high gain, low noise, low cross talk and intermodulation distortion, bit-rate transparency and polarization insensitive gain. In the development of optical amplifiers, erbium doped optical amplifier fibers have emerged as the fiber of choice because the characteristic gain bandwidth of these fibers is within a telecommunication window of 1.5 microns (1500 nm), a bandwidth commonly used in fiber optic commercial systems.
Erbium doped fibers are able to act as optical amplifiers because of their ability to exploit the energy levels of erbium, shown in FIG. 1. For example, when a photon of light, such as a 980 nm pump photon, is directed on a glass doped with Er3+, there is a high probability that the pump photon will be absorbed, exciting a ground state 4I15/2 ion to the 4I11/2 level. From the 4I11/2 level, the ion non-radiatively relaxes to the 4I13/2 level 3, releasing energy as vibrational energy, called phonons. The 4I13/2 level is metastable, possessing a lifetime of around 10 ms in silica glass. The ion in the metastable 4I13/2 level eventually emits a photon of light at around 1550 nm during fluorescence, the process whereby the excited electron of the ion radiatively returns to a lower energy level, such as the ground state. A more detailed analysis reveals that in erbium, the 4I13/2 level actually consists of seven sublevels, and the 4I15/2 consists of eight sublevels, making 56 possible transitions between the metastable and ground state.
To illustrate amplification, an Er3+ ion in the metastable 4I13/2 state can be perturbed by a 1550 nm signal photon (before it has had a chance to fluoresce). In this case, the 1550 nm signal photon stimulates the excited ion such that it emits a photon of the same wavelength, in phase, and propagating in the same direction as the stimulating photon. As the Er3+ ion returns to the ground state, there will now be two 1550 nm signal photons, the original stimulating photon and the photon emitted from the excited Er3+ ion. Amplification is achieved.
Alternatively, absorption from the ground state to the 4I13/2 state can also occur. In this case, an incoming 1550 nm signal will be absorbed, exciting some Er3+ ions in the ground state. An inversion is created as the Er3+ ion population continues to be raised to the excited state. At 100% inversion, no more ions remain in the ground state to absorb incoming photons, and an incoming 1550 nm signal will be strongly amplified.
As can be seen, gain is limited by the Er3+ ion concentration. Problems in fabrication arise when attempting to increase the Er3+ ion concentration. One problem is that of clustering, where doped Er3+ ions cluster together, destroying an individual atoms ability to generate amplification. Currently, only low Er3+ doping concentrations in SiO2 erbium doped fiber amplifiers (EDFA""s) have been achieved in optical amplifier fiber networks, with fiber lengths that exceed tens of meters. One possible solution is discussed in U.S. Pat. No. 4,075,120 to Myers et al. However, Myers only discusses the material composition of glasses with high Er3+ doping concentration for making lasers, and not fiber amplification. No phosphate erbium doped glass fiber has been commercialized for EDFA applications. This is primarily due to the inability of researchers to determine correct parameters to develop a successful workable fiber.
In addition, for amplification, compact and integrated optical amplifiers are desired in the deployment of metro and access optical networks. Known optical amplifiers are designed and assembled based on discrete active and passive optical components including. erbium doped optical amplifier fibers, laser diode modules, optical isolators, wavelength division multiplexing couplers, tap couplers, etc. Conventional amplifiers are manufactured using a box-in-a-box approach, where prepackaged devices are coupled together by splicing optical amplifier fibers, also known as fiber pigtails, in order to manufacture optical amplifiers.
Conventional optical amplifiers may be costly to manufacture and their use in optical networks may result in unwanted optical loss. Additionally, conventional optical amplifiers tend to be rather large, partially due to the fact that the erbium doped optical amplifier fiber in each optical amplifier can be up to tens of meters long. Even if the optical amplifier fiber is coiled up to save space, the bend radius of the fiber still requires a relatively large module package. In order to reduce the size of the optical amplifier module, an integrated solution is needed.
Thus, there is a need to overcome these and other problems of the related art and to provide an optical amplifier fiber, where the optical amplifier fiber is capable of commercial application. The present invention illustrated in the following description, is directed to solving one or more of the problems set forth above.
In accordance with the present invention, a compact optical amplifier module is disclosed which incorporates high gain amplifying gain mediums, such as high gain rare earth doped phosphate glass optical amplifier fibers or waveguides. Components of the optical amplifier module are optically coupled together by free space coupling, as opposed to conventional physical coupling techniques.
In an exemplary embodiment of the present invention there is an optical fiber amplifier module comprising a signal path located between a signal input and a signal output. A WDM coupler and an amplifying gain medium are optically disposed along the signal path. A pump laser which emits a pump signal is disposed out of the signal path in a manner that allows the pump signal to reflect off the WDM coupler and enter the signal path. The WDM coupler may be placed upstream of the amplifying gain medium, so that the pump signal is reflected into the upstream end of the amplifying gain medium. In an alternative embodiment, the WDM coupler is placed downstream of the amplifying gain medium, so that the pump signal is reflected into the downstream end of the amplifying gain medium. According to a third embodiment, WDM couplers may be placed both upstream and downstream of the amplifying gain medium, so that pump signals may be reflected into both ends of the amplifying gain medium.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.